The present invention relates to a system for authenticating printed documents such as bank notes.
Bank notes have many features that make it difficult to forge them. However, forgers can now recreate many of these features such as threads and watermarks. The non-fluorescent properties of bank notes have been exploited for some time by retail establishments using ultra-violet lamps, but forgers can overcome this by suppressing the fluorescence of paper used for making forged bank notes. Thus, it is now difficult for sales staff to distinguish between real and forged notes. Validation is also a problem in self service terminals such as currency deposit machines where there is no human interaction to check for forgeries. Self service terminals include automated teller machines (ATMs) which allow bank customers to withdraw bank notes, and automated currency deposit machines by which bank customers can deposit bank notes.